A division of the earth's longitudes into Zones for the convenience of measuring time in a uniform manner. Each Zone is of 15?° of longitude and accounts for 1 hour of time, East or West of Greenwich. East, the time is earlier and West time is later as the Sun rises from East and Sets to the West. i.e. 10' of longitude = 4 minutes of time. In addition, the are several special time zones which are not located at integral multiples of 15?° from Greenwich (thus, not at integral hours).